1. Field of the Invention
The present invention generally relates to a monolithic integrated circuit and, more particularly, to an analog-to-digital (AD) and digital-to-analog (DA) converter having a capability of transforming a digital signal into an analog signal in addition to a capability of converting an analog signal into a digital signal.
2. Description of the Prior Art
In the monolithic integrated circuit art, it has been customary to implement an AD converter and a DA converter as independent circuits. Various efforts have heretofore been made to promote integrated circuit configuration of an AD converter and a DA converter. Concerning an AD converter, for example, charge comparison type AD converters adopting a successive comparison principle are disclosed in "Introduction to AD Converter" by Yoneyama, 1st edition (Sept. 25, 1983), Ohm-Sha Ltd., pp. 99-109 and Japanese Patent Laid-Open Publication No. 30815/1986 by way of example.
The charge comparison type AD converter shown and described in JP Patent Laid-Open Publication No. 30815/1986 includes an array of weighted capacitors and an array of switches which are individually connected to the capacitors. While the switch array is operated, a comparator compares the varying voltage of the capacitor array with an analog input and the results of comparison are loaded in a successive comparison register. The register therefore establishes a switch connection condition of the switch array which is analogous to the analog input, outputs of the register being fed out in the form of a digital signal via a latch. This kind of AD converter is successful in reducing the geometrical area of the capacitor array and thereby the overall area required of a chip.
However, a prior art AD converter having the above configuration cannot play the role of a DA converter. More specifically, when DA conversion is needed, an independent DA converter has to be provided on the same IC substrate as the AD converter. This increases the overall dimensions of an IC chip and is therefore contradictory to the increasing demand for high integration.